A notch filter is a band elimination or band stop filter, for attenuating or damping a part of a frequency band centered on a center frequency. Stated otherwise, the transfer function of a notch filter is flat at all frequencies except for the stop band on either side of the center frequency. The drastic attenuation of the filter response around the targeted frequency is called the notch.
Notch filters have been used for years in the signal processing of signals in a reverberant or high clutter background. A biquad notch filter is a notch filter with a two pole and two zero filter topology, i.e. with a s-domain transfer function with 2 as the highest exponent both in the numerator or denominator. Such a filter is characterized by the 2 poles and 2 zeros of its transfer function. The zeroes are directly linked to the band elimination capacities of the biquad notch filter. In quadrature signaling, in-phase (I) path and quadrature-phase (Q) path need two differential twin-T RC filters.
Series LC, Twin-T RC or differential twin-T RC filters are typically used to notch undesired tones in an RF system. FIGS. 1(a), 1(b), 1(c) respectively, show a series LC filter 110, a Twin-T RC filter 120 and a differential twin-T RC filter 130. However, these conventional implementations require an integrated inductor or a considerable number of components and consequently require a significant amount of area on the silicon die.